In the realm of radar systems, there is a variety of basic technologies which are utilized to effect radar sensing systems for various applications. One basic type of radar based system is frequency modulated-continuous wave (FM-CW) radar system which emits a swept frequency electromagnetic signal at microwave or rf frequency and compares the frequency emitted with that of the frequency of the echo, resulting in a beat frequency. These FM-CW based radar sensors can be used for determination of range or in the case of a nonranging radar sensor, the systems emit a CW microwave carrier and can be used to determine velocity by the generation of the Doppler frequency shift from the moving object. Unfortunately, as is spelled out in U.S. Pat. No. 5,361,070, the disclosure of which is incorporated herein by reference, such FM-CW and Doppler radar systems based thereon have a number of drawbacks.
The aforementioned U.S. Patent to McEwan is based on a different technology to overcome some of the limitations in the FM-CW radar sensor technologies and is described herein. The system to McEwan is based on an ultra-wideband (UWB) radar motion sensing system which operates as a pulse echo system clocking the two-way time of flight of an electrical pulse of very short duration in time. The reference to McEwan is based on a carrier-free system, in which the electromagnetic radiation is a narrow pulse. The McEwan reference discloses the emission of pulses having a duration on the order of 200 pico-seconds. Such an emission results in a system having a spectrum ranging from on the order of DC to the order of a few GHz. Clearly, as is well known to one of ordinary skill in the art, the rate of change of voltage of the emitted pulse with respect to time, the first derivative with respect to time, has a direct relation to the spectral width. Accordingly, a pulse having a short duration in the time domain will have a large span in the frequency domain. This follows directly from fourier analysis. So, for example under the UWB system of McEwan, a pulse repetition frequency (PRF) signal the frequency at which the repetitive pulses of electromagnetic radiation are emitted is emitted at the transmitting antenna. With a prf of 100 Khz, with each pulse having a duration on the order of a few hundred pico-seconds, the resulting spectral distribution spans roughly 100 KHz to 3.0 GHz. While such a system enables the sensing of moving objects, there are clear drawbacks to the practical implementation of such a system.
The use of UWB non carrier based systems, such as is disclosed in the reference to McEwan, has the primary drawback of directionality or focus of the emitted beam. To this end, in most applications of proximity sensors and motion detectors, it is desired if not required that the sensing be done in a narrowly prescribed physical path. For example, were the sensor to be used for detection of objects behind a vehicle in a parking aid application, a very narrow region directly behind the vehicle would be desirably sensed for obstructing objects. Accordingly, such a sensor would require a narrowly focused or directionally focused beam for sensing objects with the radar system. Unfortunately, in a carrier-free UWB system it is very difficult to have an antenna capable of operating with the bandwidth and at the center frequency of the UWB system as is described in the reference to McEwan.
Accordingly, what is needed is a system for detection of objects both stationary and moving that has the high position resolution of a high frequency relatively wide bandwidth radar system which can be directionally focused in order to effect an accurate and reliable sensing system.